Leather articles are used in various products such as watchbands and handbags. In particular, the visual appearance of the leather enhances the commercial value of watchbands and handbags, increasing the level of satisfaction of consumers. It is needless to say that such products are designed to allow the leather to be in direct touch with the user's skin and that the feel of the leather against the skin further enhances an added value to the products.
Production of such a leather article entails making a large leather sheet in advance. In the leather production, an animal skin to be used in the leather article, such as the skin of crocodile or cow, is obtained first. The skin as obtained has low durability and cannot be used by itself. Thus, the skin is subjected to tanning. This tanning imparts heat resistance and durability to the skin, so that a leather is produced from the skin. The leather thus obtained is dyed with a desired color, or the surface texture of the leather is modified. In this manner, a sheet of leather is obtained. When this sheet is used to produce a leather article, for example, a process is performed in which the sheet is cut into a shape suitable for the intended use, and the cut sheet is attached to a core material or the like with an adhesive. Such a technique for production of leather articles has been traditionally used and is widely known.
Tanning is a technique for obtaining a durable leather by treating skin. Tannin acquired from plants had been used in the past; however, treatment with tannin fails to achieve sufficient heat resistance, flexibility, and elasticity. Recently, therefore, chrome tanning that uses a chromium tanning agent (basic chromium sulfate) and that can achieve high heat resistance, flexibility, and elasticity has become mainstream. The chrome tanning is employed for more than 90% of tanning treatments performed throughout the world and has the greatest economical importance. A hydrated chromium complex is embedded between carboxyl groups of glutamic acid and aspartic acid in a collagen peptide structure, so that a soft, durable leather is obtained. The method for chrome tanning is widely well-known and is described, for example, in Non Patent Literature 1.
High-quality leathers or leather articles with excellent heat resistance, flexibility, and elasticity are obtained typically through chrome tanning. Chrome tanning agents for chrome tanning contain chromium; thus, a large amount of chromium remains in a leather or leather article subjected to tanning using a chrome tanning agent.
Chromium is trivalent in chrome tanning agents. The trivalent chromium may be oxidized into hexavalent chromium through heating or bonding in the process of production of leathers or leather articles. Additionally, hexavalent chromium present as an impurity in chrome tanning agents may be incorporated in leathers or leather articles. Other than such hexavalent chromium incorporated through the process of production of leathers or leather articles, there is hexavalent chromium produced by the fact that trivalent chromium in the leathers or leather articles is oxidized, for example, by light, heat, or high temperature and humidity. The presence of hexavalent chromium can be examined by a measurement test. Trivalent chromium is non-toxic, while hexavalent chromium is toxic. When contacting the skin or mucous membranes, hexavalent chromium can induce skin roughness or allergy and, in severe cases, cause dermatitis or tumor. Thus, hexavalent chromium significantly affects the human body. Even a small amount of hexavalent chromium is considered to have all hazard risks including carcinogenicity, mutagenicity, and reproductive toxicity. Due to its toxicity, hexavalent chromium is specified as a banned substance.
Under the above circumstances, EU regulations on hexavalent chromium in leathers or leather articles were published as Regulations (EU), No. 3014/2014 in the Official Journal of European Union on Mar. 26, 2014. Concerning leather articles and products containing a leather in a part to be in contact with the skin, the Regulations specify that leather articles containing 3 mg/kg (3 ppm) or more of chromium(VI) oxide with respect to the total dry weight of the leather and leather part shall be restricted as of May 1, 2015 from the viewpoint of impacts on the human body (in particular, irritation to skin). The Regulations state that the method according to EN ISO 17075 is the only international standard analysis method currently available for quantification of hexavalent chromium in leathers or leather articles ((6) in the Regulations).
Under these circumstances, Patent Literature 1 states in Example 2 that placement of a drop of an aqueous solution of ascorbic acid on a tanned leather detoxified hexavalent chromium contained in the leather.